Air is a gas mixture comprising about 21 mol % oxygen, about 79 mol % nitrogen and very small amounts of other components such as carbon dioxide, argon and the like. The separation of air to provide oxygen enriched air (OEA) and/or nitrogen enriched air (NEA) is commercially significant. For example, one use of NEA is to provide an inert gas composition in the ullage of mobile fuel tanks, especially aircraft fuel tanks. Such tanks typically contain highly combustible liquid fuels of volatile organic compositions. Raising nitrogen concentration in the ullage sufficiently high can render the vapor above the liquid fuel in a fuel storage tank safely non-combustible. Nitrogen concentration can be effectively raised by displacing any ambient air in the tank with a suitable NEA.
Selectively gas permeable membranes are useful for separating components of gas mixtures. Membrane separation has been used to produce OEA and/or NEA from ambient air. Certain fluorinated composition membranes have been found effective for separating ambient air. A representative example of such a membrane has a nonporous selectively gas permeable layer of perfluorinated dioxole monomer, namely perfluoro-2,2-dimethyl-1,3-dioxole (“PDD”) copolymerized with another fluorine containing monomer such as tetrafluoroethylene (“TFE”). Such membranes are available from Compact Membrane Systems, Inc., Newport, Del. An example of a membrane separation process for separating air is disclosed in U.S. Pat. No. 5,051,114. The complete disclosure of all U.S. patents and patent applications identified in the present patent application are hereby incorporated herein by reference.
The separation performance of gas permeable membranes is typically characterized by two principal parameters, namely, the selectivity of the membrane to discriminate between components of a binary feed mixture being separated, and the permeability of the faster permeating component. Usually such membranes exhibit high selectivity with low permeability, or low selectivity with high permeability. In this disclosure reference will be made to permeance. Permeance is permeability divided by the thickness of a membrane.
The trade-off between selectivity and permeance influences the size and operating conditions necessary to achieve desired separation. For example, to obtain highly N2-enriched air at a specified volumetric rate, a very selective membrane can be used. However, the permeance of the fast-migrating component will be relatively low such that production rate is likely slower than desired. This problem can be overcome by increasing the working area of the membrane. In mobile end use applications and particularly for aircraft, size and weight have critical upper limits such that large membrane separators can be unacceptable. Similarly, smaller membranes with higher permeance can be used but enrichment per separation step is relatively low. This problem can be overcome by utilizing a train of separator steps or stages such that enrichment increases in successive steps. The drawback is that many steps or stages and auxiliary equipment, such as compressors with associated power supplies may be needed. The weight and space for the extra equipment and multiple membrane units can be prohibitive.
It is desirable to have a selectively gas permeable membrane separation system for producing NEA from ambient air that is very light weight. It is desired to have a membrane separation system that occupies a small volume and is simple to operate. There is great need for a light weight, compact, energy conserving and reliable membrane separation system suitable for generating NEA to provide inert atmosphere in the ullage of mobile tanks of volatile organic compounds, and especially of onboard fuel tanks of aircraft.